1. Technical Field
The embodiments generally relate to a guard interval length detection in an orthogonal frequency division multiplexing (OFDM) based system, and, more particularly, to non-linear quantization methods for detecting the guard interval lengths in OFDM systems.
2. Description of the Related Art
An orthogonal frequency division multiplexing (OFDM) method is known as a radio transmission method which has excellent anti-multi-path characteristic. Typically, in OFDM systems, the effect of the inter symbol interference is reduced by inserting guard intervals between OFDM symbols. Cyclic prefix, which is transmitted during the guard interval (GI), refers to prefixing of the symbol with the repetition of the end portion of the OFDM symbol.
The transmitter chooses the guard interval length based on the maximum delay spread. The majority of OFDM systems that are currently deployed have guard interval lengths specified as fractions of an OFDM symbol, where the fractions are typically negative powers of two.
The receivers use the auto-correlative properties of an OFDM symbol which are good due to a presence of a cyclic prefix to estimate the symbol and guard interval length. These estimations use the linear quantization on guard interval estimates to detect the guard interval lengths. However, the presence of echoes and lower signal to noise ratio (SNR) can yield ambiguous guard interval length estimates. Samples in the guard intervals are generally dropped in the receivers after synchronization is achieved and the Fast Fourier Transform (FFT) of the remaining samples in the OFDM symbol is subjected to channel equalization and decoding. Due to incorrect GI detection, the receiver may end up dropping the wrong samples and taking the FFT at the wrong boundary resulting in errors in the reception.
During auto-correlation, time indexes (in terms of samples) of the auto-correlation values crossing a predefined threshold are obtained. The time interval between two such threshold crossover points is an indicator of a sum of symbol length (termed as MODE) and the guard interval (GI) length. Residual length after subtracting the MODE would give an estimate of the guard interval length. Because of different echo profiles and lower SNR's obtained, the guard interval estimate may lie between two possible guard interval lengths. This would lead to ambiguity in the detection of guard interval lengths. Simple linear quantization of the guard interval estimate would lead to incorrect guard interval detection since possible guard interval lengths follow a non-linear pattern (typically powers of two).
Traditionally, a transmitter sends the information on the guard interval using the additional signaling mechanisms, for example, Transmission Parameter Signalling (TPS) signaling for DVB-T systems. The correlation is done on every symbol with FFT being taken at the correlation boundary until the TPS signals is acquired and TPS decoding is performed to extract the guard interval. This would result in increased acquisition time for the DVB-T systems.